1. Field of the Invention
The present invention relates to an electronic flash control system and, more specifically, to a control system for high-speed repeated flashing, or so-called FP flashing, during the exposure operation.
2. Description of the Related Art
Photographic flash control systems have been developed which cause multiple high speed flashes to occur during a single exposure (hereafter referred to as "FP flashing"). These multiple flashes obtain a more uniform illumination of the scene than is possible with a single long term flash. Japanese Patent Publication Number SHO 60-225833 discloses an example of such a system, wherein FP flashing is used to achieve uniform illumination during the period when the shutter is open.
With normal single flash systems, it is preferable to control the amount of light reaching the scene. Conventionally, the flash operation is stopped when the quantity of light (integration value) reached a specified value during the flash operation. For example, Japanese Patent Publication Numbers HEI 1-124838 and HEI 3-144617, show a FP flashing system wherein the on/off duty ratio of the FP flashing is changed to control the quantity of light. These systems use relatively simple schemes which initially change the duty ratio and thereafter adjust the aperture and shutter to vary exposure.
However, in recent years, the focal plane shutters of cameras have become faster, with nominal shutter speeds of 1/8000 second being reached. To obtain FP flashing which corresponds to these high shutter speeds, it is necessary to flash approximately four times in 1/8000 second to obtain a uniform quantity of light. Consequently, there is a need for a flash cycle of approximately 30 .mu.s.
With such short flash times, the actual time during which the discharge element flashes becomes shorter than the flash cycle. Further, there is a problem in integrating the quantity of light during this extremely short time and outputting a flash stop signal in time. Even if the flash stop signal is only a little late, excessive flashing will occur, causing non-uniformity in the quantity of light. Excessive flashing caused by slow or inadequate control systems will consume excessive energy from a main capacitor. This will reduce the number of repeated flashes in FP flashing, possibly to the point where repeated flashing can no longer be maintained before the completion of the movement of the rear blind of the focal plane shutter. In any event, there is a danger of producing photographs having problems resulting from non-uniformity in the quantity of light.
Current control systems have proven inadequate to perform accurate control of FP flashing during the actual flashing. For example, Japanese Patent Publication Number HEI 1-124838, discloses a system which does not measure the quantity of light and provide feedback to the duty ratio, rather it causes repeated flashing at a certain duty ratio, and performs exposure adjustment using the aperture and shutter. Thus, when the quantity of light is gradually damped, by the repeated discharge of the capacitor, the appropriate exposure could not be obtained. Adapting this system to provide such feedback and control would require a large increase in cost. Japanese Patent Publication Number HEI 3-144617 discloses a system which gradually changes the duty ratio of the high-speed repeated flashing, but does not specifically exhibit control of the quantity of light.